Tunable slot antenna assembly



March 29, 1960 A. DORNE ETAL TUNABLE SLOT ANTENNA ASSEMBLY Filed March 31, 1958 INVENTORS 427/901? DORA/E PAL PH M LOG/1W TUNABLE SLOT ANTENNA ASSEMBLY Arthur Dorne, Glen Head, and Ralph W. Logan, .West Islip, N.Y., assignors to Dome and Margohn, Inc, Westbury, N.Y., a corporation of New York Application March 31, 1958, Serial No. 725,003

21 Claims. (Cl. 343-769) This inveintion relates to the art of antennas and more particularly to a tunable annular slot antenna.

It is a particular problem in the field of antenna design to construct an antenna which has impedance characteristics matching those of the transmission line connecting it to the transmitter or receiver in order to provide for maximum power transfer. This problem is substantial in itself when the antenna is particularly designed for operation at a single frequency. However, the problem is greatly magnified when the antenna is to operate over a range of frequencies, since it is well known that as the frequency of operation changes the input impedance of th antenna will also change.

' In an antenna of annular slot design, the impedance characteristic of the aperture (slot) of the antenna con sists of two components. A resistive component is present which increases non-linearly with increase in frequency and a reactance component of capacitive nature is also present, which decreases in magnitude inversely as the frequency. In order to insure maximum power transfer from the transmission line to the antenna and to reduce any standing waves in the transmission line it is desirable to adjust the impedance characteristics of the aperture to those required for optimum operation at the frequency desired.

In order to accomplish this adjustment it is necessary that two adjustable parameters be available. If suitable adjustment range is provided it is possible to vary the impedance of the aperture over a wide range of frequencies and thereby provide a proper match to the transmission line.

The present invention provides an annular slot antenna which is suitable for operation over a range of frequencies in the ultra-frequency and microwave region, such as for example from 175 to 225 megacycles per second. The antenna itself is simple and compact and can be tuned over the required range of operation by a single control. Another advantage is its small size and low weight, permitting a size of about one-fourth the volume of a broad band fixed tuned annular slot antenna operating in the same range of frequencies.

The consequent reduction in weight and volume of the antenna is extremely desirable for use in aircraft wherein space and weight reducing features are of extreme importance.

It is therefore an object of this invention to provide a novel, tunable annular slot antenna.

A further object of this invention is to provide a tunable antenna which has a variable impedance characteristic for the aperture.

A further object of this invention is to provide a tunable antenna of novel design which is particularly suitable for use in an aircraft.

3 The present invention in its prefered form accomplishes these objects by providing a slot antenna with a built-in matching circuit coupling the antenna to a transmission line. The matching circuit has a variable-position shunt inductance which is simultaneously varied in magnitude. these'nut' holders 30. Plate 32 is of circular form; and

Patented Mar. 29, 1960 These two simultaneously adjusted parameters permits attaining an impedance match at the antenna input for a range of frequencies of operation.

Other objects and advantages of the present invention will become more fully apparent from the consideration of the following description of a preferred embodiment thereof taken in conjunction with the appended drawings, in which:

Fig. 1 shows a partially cut away view of the antenna disposed within an aircraft; and

Fig. 2 shows a cross sectional view of the structure of Fig. 1 taken along line A-A thereof.

Referring to Figures 1 and 2 of the drawings, the metallic surface of the object in which the antenna is to be housed is shown at 1. The object could be an aircraft, train, car, or even a building. The surface 1 is cut away to leave an aperture of the general shape of the antenna to be used. In this illustration an antenna of circular design is shown with an insulating cover 2 over a circular aperture in surface 1. Disposed on the inner side of the surface 1 is a metal ring 3 which is fastened to the inner side of surface 1 by any suitable means, such as rivets 4 placed around the periphery of the ring 3. As can be seen from Figure 2, the antenna is totally recessed inside its supporting object thereby eliminating any drag effects if the object is moving.

The antenna housing generally shown as 5, which forms a resonant chamber, is constructed of several portions as follows. An inner side wall 6 is provided with a flange lip 7 which is designed to test against the ring 3. A plurality of nut plates 8 are fastened by rivets 9 around the periphery of lip 7. Ring 3 is constructed with a series of holes 10 through which screws 11 can pass' and thereby fasten cover 2 to the lip 7. This secures the antenna housing 5 to the inner surface of the supporting object and it can be seen that removal of screws 11 from the outside of the object will also loosen the antenna hous the ends of which a set of angle brackets 17 are attached as by rivets 18. Bolts 19 pass through holes 20 of the brackets and are fastened by nuts 21. This serves to clamp the outer housing 16 down around the inner housing wall 6.

At the center of plate 14 a coaxial line connector having an outer conductor portion 22 is fastened to the plate 14 as by means of rivets 23. A transmission line (not shown) would run between the coaxial connector and a transmitter or receiver which the antenna serves. posed within the outer conductor portion 22 is an inner conductor pin 24. This pin 24 is insulated from an outer conductor surface 22 by a suitable insulating block 25. The center of block 25 is hollowed out to support a partially hollow metallic cone 26 which is in turn connected to pin 24. A metallic hollow cone frustrum 27 is attached to the ends of cone 26 as by rivets 23. This combination forms a conical inner surface for the atenna chamber, sometimes called a feed horn. Attached to the inner surface of the large end of feed horn wall 27 is an annular'fiange lip 29. The flange lip 29 extends completely around the inner surface of cone 27. Disposed around the periphery of flange lip 29 are a series of nut holders 30 fastened to the flange lip 29 as by rivets3l. A metallic plate 32 is fastened by means of screws 3310 Disward the ring 3, being spaced therefrom by an annular slot 34 by which the chamber defined by outer side wall 6, end wall 14 and inner conical member 27, is coupled to the space outside the supporting object. To provide for optimum radiation, the aperture 34 should be excited by a uniform radial electric field. This is best accomplished by use of a conical feed horn such as 27.which provides a more gradual transition of energy from the source (i.e. the transmission line connector) to free space on the outside of the slot aperture 34.

The annular slot antenna structure just described is generally conventional, and is highly satisfactory at ultrahigh or microwave frequencies of specific values for which the structure is designed. However, when the diameter of the antenna is small as compared with a wave length at the operating frequency used, difiiculty would be experienced in covering a broad range of frequencies. For example, in the .case where the diameter of the antenna is in the order of wave length, extreme difficulty is encountered in obtaining a satisfactory impedance match between free space and the transmission line impedance, even if the range of frequencies to be covered differs from the optimum frequency by as little as a fraction of one percent. If the antenna is of the order of /2 wave length in diameter, band widths of the order of twenty percent may be difficult to obtain. The present invention finds particular utility in cases Where the diameter of the annular slot antenna is less than /2 wave length, and where it is desired to vary the impedance characteristics of the antenna in order to obtain a proper impedance match between free space and the transmission line impedance over a relatively wide range of frequencies.

"As indicated above, such an annular slot antenna has an impedance characteristic whose resistive component increases non-linearly with frequency, and has a capacitive reactance component decreasing in magnitude in-' versely with frequency. Simple impedance-matching measures which might compensate for variations of one or the other of these components would be ineffectual as a practical matter to yield satisfactory impedance characteristics. The present invention overcomes these difficulties by simultaneously varying two separate parameters of the system to compensate for these differently varying impedance characteristics. As will be seen, this invention provides a matching circuit in which one parameter is the position of a shunt inductance while the other is the magnitude of that inductance. In a simple and convenient way, both are varied together to maintain a wide band impedance matching.

Also fastened to the outer surface of the cone 27 as by rivets 36 are a plurality of conductive vanes 35, only one of which is shown for the sake of clarity. These vanes extend radially outward from and are symmetrically spaced about the axis of the feed horn 27. While in the embodiment disclosed three vanes 35 are utilized it is understood that any desired or suitable number can be used, the actual number used being determined by the general shape of the antenna, the frequency at which it is to be operated, and other general design characteristics. Another set of conductive vanes 37 is fastened to inner housing wall 6 by means of rivets 38 passing through a flanged portion on each vane 37. The edges of vanes 35 and 37 are directly opposite and spaced from each other and these vanes are disposed in coplanar pairs around the antenna axis.

Adjacent each vane pair 35, 37, a pair of insulating blocks 39 are suitably fastened as by means of rivets 40 to metal end plate 14 and also to outer plate 32. Each of. the insulating blocks 39 is. recessed as shown. A guide rail 41 is disposed. approximately midway between the vanes and in their plane and is suitably fastened asv by means of screws 42 to the pair of insulating blocks .33. Rail. 41. can be. either of insulating or conducting material.

A metallic rod 43 or strip slides along rail 41 and has generally U shaped or spring-fingered ends 44 which make contact with vanes 35 and 37. Rod 43 may be fastened to rail 41 in any convenient Way, as by means of a clamp (not shown) tightened by a wing nut 45. Each rod 43 cooperates with one vane pair and the plurality of rods 43 can all be connected to be adjusted together along each of the rails 43 disposed around the feed horn.

The operation of the device is as follows. A coaxial cable or transmission line (not shown) is connected to coaxial connector 20. This cable will lead either to a transmitter or a receiver. As the frequency of operation changes, the aperture 34 of the annular slot antenna presents a varying impedance to the line. This would cause a mismatch and a resulting loss of transferred power. To obtain the desired impedance characteristics for the various frequencies of operation, the adjustable rods 43 are provided. These rods are effectively inductors at the frequency of operation. By moving inductor rod 43 along rail 41 the length of the electrical path through vane 35, rod 43 and vane 37 can be varied. As the rod 43 is moved towards the rear of the antenna housing the length of the electrical path increases. Thus the position of the inductance in the circuit is varied.

The opposed edges of the vanes 35, 37 are not parallel but are constructed in such a manner as to provide a greater distance between their edges at the rear end. This will mean that the effective length and therefore inductance of the rod 43 will vary as it is moved forward and backward along the rail 41. Agreater length of rod 43 and a greater inductance will be obtained towards the rear'wall 14 and ashorter length towards the front surface 32. I By means of these adjustable rods 43, the variation in input impedance of the aperture can be readily compensatedv for. Specifically, the variable inductive parameter provided by adjustable rods 43 will effectively cancel the capacitive reactance over the range of operating frequencies and thereby provide a match of antenna,

to transmission line.

over-simplified, will assist in understanding the princi ples of the present invention. In general, the electric field configuration adjacent the slot 34 is radial, so that a conductor of relatively small cross-section extending in the direction of the field has the characteristics of an inductance. The rod 43 is thus an inductance. It will be understood that it is preferably small in cross-section compared to its effective length, but may have any desired or convenient shape, such as flat, oval, circularly cylindrical, polygonal cylindrical, etc. The opposed edges of vanes 35, 37 may be considered as a transmission line section, since the electric field lines extend substantially perpendicularly between them. This line section has its outer open. end coupled across the slot 34, and is effectively inshunt with it. Also, the line section is terminated by a slidable inductance element 43. .As is well known, the variation in position of a shunt inductance will reflect varying impedances at the open end of a line section. If the operating frequency is changed, so that the slot impedance changes, a compensating. change can beeffected by adjusting the position of the shunt inductance element. By way of example, this could compensate entirely for the change in. antenna resistive component, or the change in capacitive. reactance com; ponent, but not both, over the. entire operating range, because these two components vary differently. However, by simultaneously varying the magnitude of the shunt in from front to rear. However, where more precise match ing; may be. desired, the spacing. between the vanes may vary non-linearly along their length.

As indicated above, the number of sets of vanes may be chosen as desired. In general, the more inductor rods 43 used, the greater will be the total effective shunt inductance, so that this parameter may be determined both by the shape of the individual inductor rods 43 and the number used.

It has been found that making the edge of the inductor rod 43 parallel to the front face 32 of the antenna will provide for tuning over the widest range of frequencies. This can be accomplished by slanting the rod clamp in a suitable manner. However, the embodiment shown wherein the edge of the rod is perpendicular to the axis of the rail 41 also produces excellent results.

It will be understood that any suitable mechanism may be used for simultaneously adjusting the several inductor rods 43, and the details of these are within the skill of any mechanic. For example, each rod 43 may be connected to a push rod shown as 46 extending out the back, with the several pushrods being unitarily controlled. Many other equivalent arrangements m-ay be readily conceived and require no illustration here.

While guide rails 41 form one convenient way to permit adjustment of inductor rods 43, it will be understood that the present invention is not limited thereto, since any other desired arrangement for adjusting the rods 43 may be utilized.

It is obvious that many changes could be made in the above construction and many widely differing embodiments of this invention could be made without departing from the scope thereof. It is intended that all matter and description contained in the above description or accompanying drawing shall be interpreted in an lllUSe trative and not a limiting sense.

What is claimed is:

l. A tunable slot antenna comprising a chamber with an aperture, said aperture having a varying impedance characteristic over a range of frequencies, an impedance element disposed Within said chamber, and means for simultaneously varying the impedance of said impedance element and the distance of said impedance element from said aperture.

2. In a tunable antenna including -a chamber containing an aperture whose impedance characteristics vary with a change in frequency, an impedance element and means for simultaneously varying the magnitude of impedance of said element and the distance of said element from said aperture to compensate for the variations of aperture impedance with frequency.

3. A tunable slot antenna comprising a chamber with an aperture, said aperture having a varying impedance characteristic over a range of frequencies, a plurality of impedance elements disposed within said chamber, and means for simultaneously varying the magnitude of impedance of said elements and the distance of said elements from said aperture.

4. In a tunable antenna including a chamber containing an aperture whose impedance characteristics vary with changes in frequency, a plurality of impedance elements, and means for moving said impedance elements to simultaneously vary the magnitude of their impedances and the distances of said elements from said aperture to compensate for the variations of aperture impedance with frequency.

5. In a tunable antenna including a chamber containing an aperture whose impedance characteristics vary with changes in frequency, a plurality of impedance elements, means for moving said elements together to simultaneously vary the magnitude of their impedances and the position of said elements with respect to said aperture to compensate for the variations of aperture impedance characteristics with frequency.

6. A tunable antenna comprising a chamber having an aperture, a pair of vanes oppositely disposed within said chamber, said pair of vanes forming part of a conducting path, an impedance element completing said conducting path, and means for moving the impedance element to simultaneously vary its position and' magnitude of its impedance.

7. An antenna as described in claim 6 wherein the edges of the vanes opposite one another are not parallel.

8. A tunable antenna comprising a chamber having an aperture, a plurality of pairs of vanes oppositely disposed within said chamber, each of said pairs of vanes forming part of a conducting path, a respective impedance element completing each of said conducting paths, and means moving said impedance elements to simultaneously vary the lengths of said paths and the magnitudes of the impedances of said elements.

9. An antenna as described in claim 8 wherein the opposite edges of each pair of vanes are not parallel to each other.

10. A tunable antenna comprising a chamber having an aperture, a plurality of pairs of vanes angularly symmetrically spaced within said chamber, each of said pairs of vanes forming part of a conducting path, a respective impedance element completing each of said conducting paths, and means for moving said elements to simultaneously vary the lengths of said paths and the magnitudes of the impedances of said elements.

11. An antenna as described in claim 10 wherein the opposite edges of each pair of vanes are not parallel to each other.

12. A tunable slot antenna comprising a resonant chamber having front, rear and side walls, and an aperture in said front wall, means connecting a tapered member between said front and rear walls, a plurality of pairs of vanes, means connecting one of the vanes of each of said pairs to the tapered member and means connecting the other of the vanes of each of said pairs to the side walls, the vanes of each pair being coplanar, means fastening a plurality of rails to said front and back wall each substantially in the plane of a respective pair of said vanes, a rod fastened to each of said rails with each rod in electrical contact at either end with the vanes of a respective vane pair, and means to move said rods along said rails for simultaneously varying the magnitudes and positions of the impedances presented by the rods.

13. A tunable slot antenna comprising, a resonant chamber having front, rear and side walls, and an aperture in said front wall, means connecting a tapered member between said front and rear walls, a plurality of pairs of vanes, means connecting one of the vanes of each of said pairs to the tapered member and means connecting the other of the vanes of each of said pairs to the side walls the vanes of each pair being coplanar, said pairs being angularly symmetrically spaced about the center axis of said tapered member, means fastening a plurality of rails to said front and back wall each substantially in the plane of a respective pair of said vanes, a rod fastened to each of said rails, with the longitudinal axis of the rod parallel to the front wall and with each rod in electrical contact at either end with the vanes of a respective vane pair, and means to move said rods along said rails for simgltaneously varying the magnitudes and positions of the impedances presented by the rods.

14. A tunable slot antenna comprising a chamber having front, rear and side walls, and an aperture in said front wall, means connecting a tapered member between said front and rear walls, a plurality of pairs of vanes, means connecting one of the vanes of each of said pairs to the tapered member and means connecting the other of the vanes of each of said pairs to the side walls the vanes of each pair being coplanar, said pairs being angularly symmetrically spaced about the center axis of said tapered member, and the edge of each vane being opposite but not parallel to the edge of the other vane of its pair, means fastening a plurality of rails to said front and back wall each substantially in the plane of a respective pair of said vanes, a rod fastened to each of said rails with each rod in electrical contact at either end with the vanes of a" respective vane pair, and means to move said rods together slidably along said rails for simultaneously varying the magnitudes and positions of the impedances presented by the rod.'

15. An antenna as described in claim 13 wherein the longitudinal axis of the rod is parallel to the front of the chamber.

16. A tunable annular slot antenna comprising a conductive housing having side and rear walls, a front plate spaced from said side walls and forming an annular radiating slot therebetween, means for coupling a transmission line between said housing and said front plate, and adjustable impedance-matching means within said housing, said means comprising a variable magnitude impedance element movable in distance relative to said slot.

17. A tunable annular slot antenna comprising a con cluetive housing having side and rear walls, a front plate spaced from said side walls and forming an annular radiating .slot therebetween, means for coupling a transmission iine between said housing and said front plate, and adjustable impedance-matching means within said housing, said means comprising means defining effectively a transmission line section within said housing'and-coupled across said annular slot, an impedance element connected across said line section, and means for simultaneously varying the impedance value of said element and the distance of said impedance element relative to said slot.

18. A tunable antenna comprising a chamber having an aperture, a pair of opposed edge vanes in said chamber end coupled on either side of said aperture, and an adjustable inductor element extending between said vanes.

19. An antenna as set forth in claim 6 wherein the aperture is substantially annular. l 20. An antenna as set forth in claim 10 wherein th aperture is substantially annular.

21. An antenna as set forth in claim 13 wherein the aperture is substantially annular.

References Cited in the file of this patent UNITED STATES PATENTS Lindenblad Oct. 30, 1951 2,644,090 Dorne June 30, 1953 2,699,501 Young et al. Jan. 11, 1955 

